A. Field of the Invention
Embodiments of the present invention are related to memory storage devices and computing devices employing such memory storage devices. In particular, the present invention relates to magnetic random access memory (MRAM) storage devices based on multiferroic tunnel junctions.
B. Description of the Prior Art
Spintronics is a field of electronics which aims at utilizing the spin of the electron in addition to its charge for memory and logic applications. Due to advancements in spintronics, information storage has experienced tremendous growth in the past decade. In particular, magnetic random access memory (MRAM) is an emerging technology which could provide a non-volatile and energy-efficient alternative to the traditional dynamic random access memory (DRAM). In MRAM memory bits the state is stored in the relative orientation (parallel or antiparallel) of the magnetization of two ferromagnetic (FM) thin films separated by a thin non-magnetic insulator. Such devices are referred to as magnetic tunnel junctions (MTJs) which have the unique and important property of having their resistance change as the relative orientation of the magnetizations switches from parallel to antiparallel. This phenomenon, referred to as the tunneling magnetoresistance (TMR), is the foundation of MRAM applications. The TMR is defined by, TMR≡(IP−IAP)/IAP, where IAP (IP) is the current for the antiparallel (parallel) relative magnetization orientation of the two FM films (W. H. Butler, et al., Phys. Rev. B 63, 054416 (2001). S. S. P. Parkin, et al., Nat. Mater. 3, 862 (2004); S. Yuasa, et al., Nat. Mater. 3, 868 (2004).).
Switching the magnetization orientation is normally achieved by an external magnetic field which hinders the miniaturization of MRAM and is not energy efficient. An alternative mechanism involves driving a spin-polarized current through the device which induces a spin transfer torque (STT) (J. C. Slonczewski, Phys. Rev. B 39, 6995 (1989). H. Kubota, et al., Nature Phys. 4, 37 (2008); J. C. Sankey, et al., Nature Phys. 4, 67 (2008).)
The STT is the transfer of spin angular momentum between the non-collinear magnetizations of the FM layers. Even though this mechanism simplifies the device design it still requires high electric currents for magnetization switching which, in turn, result in high power consumption.
Recently, ferroelectric tunnel junctions (FTJs), consisting of metal electrodes with a thin ferroelectric (FE) barrier, have been shown to change resistance in electric field (M. Y. Zhuravlev, et al., Phys. Rev. Lett. 94, 246802 (2005); J. P. Velev, et al., Phys. Rev. Lett. 98, 137201 (2007). Zhuravlev, et al., Phys. Rev. B 81, 104419 (2010). V. Garcia et al., Nature 460, 81 (2009); P. Maksymovych, et al., Science 324, 1421 (2009). J. P. Velev, et al., Nano Lett. 9, 427 (2009). V. Garcia, et al., Science 327, 1106 (2010).).
The effect described above in paragraph [0006] is called tunneling electroresistance (TER) and is defined by TER≡(I→−I←)/(I→+I←), where I←(→) is the total charge current when the polarization points to the right (left). The underlying mechanism lies in the asymmetric screening of the polarization bound charge at the two metal/FE interfaces upon switching the ferroelectric polarization in the barrier. Moreover, multiferroic tunnel junctions (MFTJs), which can be thought alternatively as MTJs with FE barriers or as FTJs with FM electrodes, show simultaneous TER and TMR effects. More importantly in MFTJs the electric control of the spin-polarized charge current and TMR is realized via the dependence of the current on the polarization direction in the barrier.
The digital era requires electronic memory storage that is reliable, quick, energy efficient, and appropriately sized for the application. Current MRAM storage devices require high electric currents, and therefore tend to be not very energy efficient. Also, it tends to be difficult to make efficient, reliable MRAM devices that are small enough for efficient use in various applications.
Thus, a need exists in the art for improved MRAM storage devices and associated methods and apparatus.